A modern communications system widely uses multiple antennas to improve a system capacity and coverage, and improve user experience. For example, a Long Term Evolution (LTE) R8 system in the 3rd Generation Partnership Project (3GPP) can support four antenna ports, while an LTE R10 system can support eight antenna ports. By using a transmit beam forming (BF) or precoding technology and by using a receive signal combination technology, a multiple input multiple output (MIMO) system can obtain a diversity and array gains. A received signal in a system that uses the BF or precoding technology may be generally denoted by:y=Hvs+n, where
y denotes a vector of a received signal, H denotes a channel matrix, V denotes a precoding matrix, s denotes a transmit symbol vector, and n denotes measurement noise.
Precoding generally requires a transmitter to fully learn channel state information (CSI). A commonly used method is that user equipment quantizes instantaneous CSI and feeds back the instantaneous CSI to a base station. CSI information fed back by the existing LTE R8 system may include a rank indicator (RI), a precoding matrix indicator (PMI), a channel quality indicator (CQI), and the like. The RI and the PMI are used to indicate a serial number of a used layer and a used precoding matrix respectively. Generally, a set of used precoding matrixes is referred to as a codebook, and each precoding matrix in the set may be referred to as a codeword. A 4-antenna codebook in the existing LTE R8 system is designed based on a Householder transformation, and a dual-codebook design is further introduced into the LTE R10 system for the eight antennas. The foregoing two codebooks are primarily pertinent to an antenna design of a conventional base station, where the conventional base station controls an antenna beam direction in a vertical direction by using a fixed downtilt angle or a remote-electrical-tilt downtilt angle, and only a beam direction in a horizontal direction can be adjusted dynamically by means of precoding or beamforming.
To reduce system expenditure and accomplish a higher system capacity and meet a higher coverage requirement, an active antenna system (AAS) has been widely deployed in practice. In addition, an LTE R12 system that is currently launched and a future LTE R13 system are considering communication performance to be enhanced after an AAS system is introduced.
Different from the conventional base station, an AAS base station further provides freedom in designing the vertical direction of the antenna, which is primarily implemented by a two-directional antenna array in the horizontal and vertical directions of the base station. For the conventional base station, although each antenna port in the horizontal direction of the conventional base station can be obtained by weighted combination of multiple array elements in the vertical direction, the conventional base station actually uses only a horizontal one-dimensional array. For example, FIGS. 1-1A is a schematic diagram of a uniform linear array (ULA) antenna configuration of a conventional base station, FIGS. 1-1B is a schematic diagram of a cross polarization (XPO) antenna configuration of the conventional base station, FIGS. 1-2A is a schematic diagram of a uniform linear array antenna configuration of an AAS base station, and FIGS. 1-2B is a schematic diagram of a cross polarization antenna configuration of the AAS base station. In addition, for the AAS base station, more antenna ports may need to be considered. For example, the currently considered quantity of ports may be 8, 16, 32, or 64. Moreover, even with a same quantity of antenna ports, structures of the antenna arrays may differ. Therefore, a different channel state measurement result may be obtained for an antenna port of a same serial number in a different array structure. For example, as shown in FIGS. 1-2A, an antenna array A is a 2-row 8-column uniform linear array, and an antenna array B is a 4-row 4-column uniform linear array. Although the antenna array A and the antenna array B each have 16 antenna ports, the structures of their antenna arrays differ.
In addition, it should be specially pointed out that backward compatibility is an important factor to be considered in designing a new LTE R12 system or an LTE system of a higher version. For example, it is required that the LTE R12 system equipped with an AAS base station can ensure normal operating of UE in LTE R8-R10 systems or can prevent deterioration of operating performance of the UE. In this background, a new design scheme needs to be proposed about how to measure and report channel state information, so as to improve efficiency or performance of a communications system.